Design Guideline

PCB Design Guideline and Manufacturing Input Requirement

Better the designing and better the PCB manufacturing!

It is observed that PCB designers are concentrating their attention on electronic parameters. Keeping eye on a large number of components available in the market, selecting the correct component and updating the component library they forgot to pay attention on production capacity of their supplier.

If possible avoid data in Gerber RS274-D format as it is already obsolete format. If you send data in gerber RS274-D format then make sure aperture file is there and with the apertures used in the design.

Where possible check your generated output data (Gerbers & Excellon) with a Gerber viewer before you send it on to production. Make sure that all instructions or other necessary input needed for making the boards are included in the Gerber and Excellon files.

Use clear and easy to understand file naming and try to avoid long filenames. Make sure that we can easy determine the layer function from the filename or add readme file for file identification. Avoid too long file naming.

Do not scale your data. All data provided must be scale 1/1 (100%).

Make sure that your Gerber files do not contain apertures with a zero-size (size =
0.00mm) and that your Excellon data does not have zero-sized tools (size=0.00mm).

Use the same units (mm or inch) in your Gerber & Excellon output files as in your CAD PCB design software. This will eliminate conversion or rounding errors.

Use the same resolution (grid) for your Gerber & Excellon data to allow a perfect match.

Make sure that your data is supplied as seen from top to bottom through the PCB. Do not mirror (or reflect) any data layer – image or drill. Viewing a PCB from top to bottom through the board is the universal practice in the PCB industry. This means that when viewing your Gerber data, text on the top side of your board (copper, soldermask, legend) should be readable and text on the bottom side (copper, soldermask, legend) should be non-readable (= mirrored).

Drill Layer:

Do not overlap drill holes. These can cause broken drill bits and the small pieces of material left behind may cause voids in the plated hole barrels.

Do not use overlapping drill holes to define slots.

All PCB drills are manufactured in step of 0.05 mm so wherever possible combine the drill size that are within 0.05mm with each other.

For PTH holes on the board edge, clearly mention for it in the mechanical layer.

If possible, do not overlap NPTH holes on the board contour / outline, consider it as a part of outline and draw the correct outline.

Copper Layers:

When generating output, use “flashes” for pads and avoid “drawn pads” (i.e. pads filled with small draws)

Drawn Pads

Avoid drawn planes, filling large copper areas or copper planes with small draws (“painting”). Where possible, use contours or polygons to construct areas or planes. Contourized areas or Polygon Area fill are standard features in Extended Gerber output (RS-274X).

Drawn Pads

Include the board outline in your copper layers. This will ensure proper alignment of all layers in case of an offset problem. Also include the board outline in a separate Gerber mechanical plan.

Remove copper pads from NPTH holes if these pads are not used or connected to other copper.

Keep copper pattern away from the PCB edge as per your manufacturer’s specification.

If you require a copper area or plane to extend up to the board edge, then clearly indicate this in the mechanical layer.

If you require the board edge or part of it to be plated, then clearly indicate this in the mechanical layer.

Use top viewed right-reading text to label every gerber layer to prevent mirror board production.

Avoid peelables & same net spacing errors.

Use thieving and copper balancing to the outer layers to ensure even plating.

Use full copper rather than hatch patterns in copper planes.

ALWAYS provide the proper layer sequence for a multilayer board. The layer
sequence can be given in several ways: Indicate the layer numbers in the copper
image by placing a logical number in each layer (1 for top layer, 2 for inner1, 3 for
inner2, etc….).

If the plane layer design leaves strips of copper between isolated pads, a minimum of 0.20mm is required between isolated pads to avoid causing shorts due to resist lifting and redepositing.

Make sure your thermal relief pads are properly defined and comply with the production requirement for Annular Rings (AR), Track Widths (Thermal Segment Width) and Gaps. Good practice for Thermals is to work with a Gap of 0.25mm (10mil) and a Thermal Segment Width of 0.20mm (8mil).

Avoid thermal isolation.

To avoid bow and twist problem, board must have a balanced construction. It is good practice to add redundant layers in order to make the board symmetrical from the centre line of the total thickness.

Number and weight of copper layers should be mirror imaged about the centre of the
package.

Avoid low pressure area. Due to low pressure area, entrapped air in the laminated panel do not allow flow of air and resin and it causes micro-voids in laminated multilayer boards or may cause delaminating problem. Add venting or copper pattern in inner layers to equalize pressure distribution.

Solder Mask and Legend:

Typically solder mask should have a minimum of a 0.10 mm “growth/mask opening” around the pad to allow for any mis-registration.

The minimum solder mask “dam” should be 0.10 mm (a dam is the minimum track size of solder mask). Once the “dam” goes below 0.10 mm, it becomes more difficult to process without the “dam” lifting off the PCB.

For Legend print, the main consideration is font size / height. Font height less than 1 mm is eligible after printing.

The legend print will ALWAYS be broken – or clipped – against the corresponding solder mask layer to make sure legend ink doesn’t come on pad for better soldering.

Remove any bits of line smaller than 0.15mm (6mil).

In absence of a solder mask layer, the legend print is to be clipped against the corresponding copper layer. If there is no copper layer, the legend is to be clipped against the drill layer.

To avoid legend being clipped maintain a minimum distance of 0.20mm (8mil) between your legend elements and the copper image.

Mechanical Layer:

A mechanical layer should minimally contain:

The exact board outlines, ideally including dimensions (mm or inch).

Exact positions and sizes for all inside milling, slots or cutouts, ideally including dimensions (mm or inch).

While giving dimensions, always take the center of the contour lines to be the exact outline of the board, regardless of their thickness.

Additional information that should be included in the mechanical plan when needed:

A reference hole: the distance from one drill hole in X and Y to the PCB outline. This is particularly important when you only have NPTH holes without copper pads.

Positional indication of all drills using symbols (=drill map). Use different symbols for each different drill size.

PTH/NPTH indication for holes and slots.

If you supply panelized data include proper indications for break-routing and/or scoring (V-cut).

A clear layer sequence or buildup drawing including all copper layers, soldermask and legend layers, any additional layers like peel-off or carbon, in the correct sequence viewed from top to bottom and with the correct corresponding data file name.

If the PCBs are to be delivered as an array for assembly, then the mechanical drawing for the array should be included as well. The overall tolerances for mechanical dimensions are +/- 0.25mm.

Carbon:

Always clearly indicate on which side of the PCB carbon is to be applied. This can be on one side or on both sides. Indicate the position by proper file naming and a clear buildup or layer sequence description in the mechanical layer.

When generating output include the board outline in your carbon layer(s).

Generally the peel-off mask is applied ONLY to 1 side of the PCB, usually the bottom side.

If you require Peel-off on the top side of the PCB or on both sides, please CLEARLY indicate this by means of proper file naming and clear buildup or layer sequence description in the mechanical layer.

When generating output include the board outline in your peel-off layer(s).

Avoid using many different small peel-off areas randomly placed on your PCB. Make the peel-off area as large as possible by connecting the separate peel-off areas wherever practical. This will make it easier to remove the peel-off after soldering.